1. Field of the Invention
This invention relates to a method for detecting image signal components, which represent an artifact due to dust, a scratch, or the like on a radiation image recording medium, from an image signal made up of a series of image signal components obtained during an image read-out operation carried out on the recording medium, such as a stimulable phosphor sheet or X-ray film, on which a radiation image has been recorded.
2. Description of the Prior Art
Techniques for reading out a radiation image, which is recorded on a recording medium, in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields.
For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on a sheet of X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film with a microphotometer, or the like, and converted into an electric signal, and the electric signal (image signal) is processed and then used for reproducing the X-ray image as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used to reproduce the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT), or the like.
During the recording of a radiation image on a recording medium, such as a stimulable phosphor sheet or X-ray film, if dust clings to the surface of the recording medium, the dust will prevent radiation from impinging upon the recording medium. Therefore, when an image signal is detected from the recording medium, on which a radiation image has thus been recorded, during an image read-out operation, and a visible image is reproduced from the image signal, a white dot-like artifact will occur at part of the visible image, which part corresponds to the part of the recording medium at which the dust was present. Particularly, in the radiation image recording and reproducing systems using stimulable phosphor sheets, if dust clings to the surface of a stimulable phosphor sheet, on which a radiation image has been stored, during an image read-out operation, the dust will prevent stimulating- rays from impinging upon the stimulable phosphor sheet. Therefore, the part of the stimulable phosphor sheet at which the dust is present will not be substantially caused to emit light in proportion to the amount of energy stored thereon during its exposure to radiation. Accordingly, when a visible image is reproduced from the image signal obtained during the image read-out operation, a white dot-like artifact will occur at part of the visible image, which part corresponds to the part of the stimulable phosphor sheet at which the dust was present. Also, in the radiation image recording and reproducing systems using stimulable phosphor sheets, if the surface of a stimulable phosphor sheet has a scratch, a flaw, or the like, stimulating rays irradiated to the stimulable phosphor sheet during an image read-out operation will be scattered by the scratch, the flaw, or the like. As a result, a white dot-like artifact will occur at part of a visible image reproduced from an image signal obtained during the image read-out operation, which part corresponds to the part of the stimulable phosphor sheet at which the scratch, the flaw, or the like, is present.
In cases where reproduced radiation images are utilized in diagnoses of human bodies, or the like, white dot-like artifacts very adversely affect accurate and efficient diagnoses. For example, in the case of a radiation image of the mamma, or the like, calcium salts which have deposited in a tissue will appear in the form of a white dot in a reproduced visible image. Such an effect is utilized in detecting a cancer, or the like. Therefore, if a white dot caused to occur in a reproduced visible image by the deposition of calcium salts (i.e. calcification) and a white dot-like artifact due to dust, a scratch, or the like, are discriminated from each other, a diagnosis cannot be carried out accurately. Of course, the size of a white dot-like artifact due to dust, a scratch, or the like, varies in accordance with the size of the dust, the scratch, or the like. It often occurs that the size of a white dot-like artifact is approximately equal to the size of a white dot caused to occur by calcification.
In order to eliminate the problems described above, various processes for preventing dust from clinging to recording media and preventing recording media from being scratched or flawed have heretofore been employed. For example, U.S. Pat. No. 4,703,537 discloses a method wherein dust is removed from a stimulable phosphor sheet by a cleaning roller.
However, considerable labor is required and it is actually difficult completely to prevent dust from clinging to recording media and to prevent recording media from being scratched or flawed.
If image signal components representing a white dot-like artifact due to dust, a scratch, or the like on a recording medium can be detected accurately from an image signal made up of a series of image signal components obtained during an image read-out operation carried out on the recording medium, such as a stimulable phosphor sheet or X-ray film, on which a radiation image has been recorded, it would become possible to prevent an artifact from occurring in a visible image reproduced from the image signal by carrying out an appropriate processing on the image signal components representing the artifact.